ISDN offers customers a digital connection to their telephone service provider, using the same phone lines that are used for conventional analog phone service. FIG. 1 illustrates ISDN Basic Rate Interface (BRI) service, which operates across a single twisted pair local loop 22 (the U interface in ISDN terminology). One end of local loop 22 terminates at the telephone service provider's ISDN switch 24, typically located at a central office. ISDN switch 24 connects in turn to network 26, allowing ISDN customers to connect, e.g., calls to circuit-switched telephone equipment 28, or data connections to packet-switched data terminal equipment 30.
At the customer's end of local loop 22, a network termination 32 converts between the U interface signal and a four-wire S/T interface digital signal (a T interface is point-to-point, while an S interface is point-to-multipoint). An S interface 34 is shown connected to several types of terminal equipment. ISDN telephone 36 is provisioned for direct connection to S interface 34. Router 38 is also provisioned for communication over S interface 34, and connects, e.g., to local area network 40. Terminal adapter 42 formats conventional analog circuit-switched signals, e.g., from an analog telephone 44 or an analog computer modem 46, to an ISDN digital carrier format. Other types of terminal equipment, such as PBX equipment, credit card authorizers, etc., (not shown) may also connect to S interface 34.
ISDN BRI service provides three bi-directional channels on an ISDN connection. Two of these channels are B (bearer) channels, B1 and B2, and the third channel is a D (delta) channel. Each B channel has a 64 kbps (thousand bits per second) data rate, and can be used for voice or data. Thus at any given time, an ISDN BRI can support, e.g., two voice calls, or a voice call on B1 and packet data transfer on B2, or 128 kbps packet data transfer using both B1 and B2. The D channel has a 16 kbps data rate, and is used for exchanging call set up and signaling messages for the B channels. The D channel can also be used to send and receive X.25 packet data.
One disadvantage of ISDN is that setting up and configuring terminal equipment can often be a daunting task for a novice. For instance, when equipment is physically connected to an ISDN line, the basic layer 2 transport mechanism is enabled. But most useful communications with the provider's ISDN switch will be disabled unless the terminal equipment knows its Service Profile Identifier (SPID), which is usually its 10-digit directory number with or without other strings of digits concatenated with it. It is also useful if the terminal equipment knows the type of ISDN switch it is connected to, since each type of switch may have certain functional peculiarities that can be compensated for by the terminal equipment. Finally, there are also a range of secondary data and/or voice services/features that may or may not be implemented on this particular ISDN switch and/or line.
Recent advances in ISDN have created a mechanism whereby terminal equipment can automatically obtain a list of valid SPIDs from the switching system, allowing the equipment to auto-configure its SPID. An Automatic Switch Detection process also now allows terminal equipment to determine the type of ISDN switch (e.g., 5ESS, DMS-100) that it is connected to. But no mechanism currently exists for automatically determining which, if any, secondary services/features are available on the ISDN line.